Transient absorption spectroscopy of the iron(II) [Fe(phen) 3] 2+ complex: Study of the non-radiative relaxation of an isolated iron(II) complex

[Display omitted] ► We studied optical excitation of [Fe(phen) 3] 2+ complex in solution. ► Upon its excitation from the 1A 1 ground state, the complex is promoted towards its 1MLCT singlet excited state. ► It relaxes towards its 3MLCT and 5T 2 quintet states. ► The ensemble of these processes lasts ∼9 ps. ► In ∼1.1 ns, the complex relaxes back towards its 1A 1 ground state. We report on the light induced spin crossover dynamics of [Fe(phen) 3] (BF 4) 2 (phen = 1,10-phenantroline) complex dissolved in acetonitrile. This complex was excited by picosecond and femtosecond laser pulses whose central frequency was centered on the 1A 1 → 1MLCT optical transition. Our analysis that well reproduces the recorded transient absorptions suggests this complex relaxes following inter-system crossing cascade: 1MLCT( S = 0) → 3MLCT( S = 1) → 5T 2( S = 2) → 1A 1( S = 0). The 3MLCT and 5T 2 states were identified, thanks to the triplet–triplet 3MLCT → 3MLCT ∗ and 5T 2 → 5MLCT optical transitions. Our experiments well agree with the relaxation mechanism recently proposed and clearly indicate the impact of the solvent on the measured relaxation constant time.